Heavy hydrocarbons, e.g. bitumen, represent a huge natural source of the world's total potential reserves of oil. Present estimates place the quantity of heavy hydrocarbon reserves at several trillion barrels, more than 5 times the known amount of the conventional, i.e. non-heavy, hydrocarbon reserves. This is partly because heavy hydrocarbons are generally difficult to recover by conventional recovery processes and thus have not been exploited to the same extent as non-heavy hydrocarbons. Heavy hydrocarbons possess very high viscosities and low API (American Petroleum Institute) gravities which makes them difficult, if not impossible, to pump in their native state.
Various different methods have been developed for recovering heavy hydrocarbons such as bitumen. Cold production techniques include mining, natural depletion, cold heavy oil production with sand (CHOPS) and vapour extraction (VAPEX). Thermal production techniques include steam assisted gravity drainage (SAGD), cyclic steam stimulation (CSS) and in situ combustion (ISC). SAGD and CSS both utilise steam to heat up hydrocarbon thereby reducing its viscosity to render it mobile. The use of steam in ISC is also common wherein it is often employed to heat up the formation to a high enough temperature to enable combustion to be initiated.
The method that is used most often commercially today for heavy hydrocarbon recovery from subterranean reservoirs is SAGD. In this method two horizontal wells are drilled approximately five meters vertically apart and steam, typically generated in a once through steam generator, is injected into the formation through the upper wellbore. The steam permeates the formation and reduces the viscosity of the heavy hydrocarbon (e.g. bitumen) present therein thereby enabling it to flow from the reservoir and into the lower well. From there it is pumped to surface facilities.
The mobilised hydrocarbon recovered at the surface is in the form of a mixture with water from condensed steam and formation water. Various minerals and inorganic salts, e.g. silica, iron, carbonates, are also dissolved or suspended in the mixture. These may, for example, derive from the water and/or the formation.
When the mobilised hydrocarbon is collected at the surface it is usually treated to separate the produced hydrocarbon from the water. Typically the water that is obtained from this separation process is recycled by using it for the generation of further steam in a steam generator. Usually, however, the water must first be purified to render it suitable for feeding to a boiler for steam generation. Otherwise the minerals and inorganic salts present in the water precipitate out to form deposits that stick to the heat surfaces of the boiler in a process often referred to as “fouling”. The deposits form a thermal barrier on the heat surfaces and increase the temperature of the surfaces which ultimately reduces the strength of their material and their service lifetime. The deposits also reduce the heat transfer to water to generate steam thus reducing the quantity and quality of the steam subsequently produced by the steam generator. Boilers generally need to be taken out of operation at regular intervals for cleaning and maintenance to remove deposits created by fouling. The higher the degree of fouling the shorter the operational periods between cleaning and maintenance are.
To minimise the amount of fouling that occurs in a once through steam generator the quality of steam produced therefrom is usually limited to around 80% by weight. The presence of 20% water in liquid form in the steam means that drying out at the heat transfer surface is less likely to occur. As a result, the precipitation of solids on the pipe surfaces may be minimised. On the other hand, however, this means that 20% by weight of the water that is fed into the boiler is not converted to steam which represents a significant inefficiency. Moreover this water must ultimately be treated for recycling to the boiler or for disposal.
Various different methods may be employed to treat water recovered from hydrocarbon production and/or steam generation prior to recycling it for steam generation. Chemical means can, for example, be used to reduce water hardness and silica content. Evaporation can also be used although this is energy intensive. The effective treatment of large volumes of separated water from hydrocarbon recovery operations economically is therefore a challenge.
Additionally or alternatively the accumulation of minerals and salts on the heat transfer surfaces of a steam generator may be prevented or minimised by “blowing out” or “blowing down” a portion of the water produced from the hydrocarbon recovery operation and obtained in the hydrocarbon/water separation step. In this case the water for steam generation is usually supplemented with fresh water. The combined affect of consuming fresh water and blowing out waste water, e.g. to deep groundwater reservoirs, is significant. Even with care, it has an environmental impact.
A major problem associated with the use of steam in recovery of heavy hydrocarbons is therefore the supply of suitable water for steam generation in a steam generator, typically a once through steam generator. Fouling of steam generators leads to short service periods and high maintenance costs and reduces the quality of steam produced. The treatment of water from previous recovery operations to make it useable for steam generation is, however, costly and only partially effective. The use of large amounts of fresh water has a significant environmental impact as underground fresh water reservoirs are depleted and blow out water is stored underground.
There have been various different strategies developed to try to overcome these problems. US2009/0133643, for example, describes a method for steam generation for injection into a hydrocarbon reservoir that reduces the amount of boiler blowdown that requires treatment and/or disposal. In this method the blowdown from a first once through steam generator is fed directly (i.e. without purification) to a so-called blowdown boiler that produces further steam. The output of the blowdown boiler is dry saturated steam and a blowdown stream of reduced volume compared to that produced from the first steam generator. This therefore increases the total amount of steam produced from a given volume of water and correspondingly reduces the amount of blowdown for actual disposal. The once through steam generators used in US2009/0133643 and the conditions in which they are operated seem to be entirely conventional. Despite this US2009/0133643 states that its configuration does not lead to rapid fouling of the blowdown boiler as would be expected to result from the introduction of water comprising minerals and inorganic salts. No reason or explanation for this result is provided.
US2011/0017449 discloses a different approach wherein a once through steam generator is adapted to operate with high impurity water to provide steam having a quality of at least 80% by volume. The key to the methods disclosed in US2011/0017449 is the use in its once through steam generator of pipes having a bore with an inner surface having ribs that define a helical flow passage. The helical flow passage guides the water though the pipes, imparting a swirling motion thereto, which controls the concentrations of the impurities in the water. This reduces the likelihood that droplets of water form and avoids the generation of droplets having high concentrations of impurities that are prone to precipitation in the form a deposit, i.e. to fouling. As a result, water having much high levels of impurities can be used as a water supply for the once through steam generator.
On the other hand, however, this method requires the production of special pipes having the necessary interior configuration and then their incorporation into steam generators. Such modifications are significant and not easily undertaken. These pipes cannot be retrofitted into a boiler, rather they have to be installed by the manufacturer at the time the boiler is made.
A need therefore still exists for alternative methods for overcoming the problem of generating steam for recovery of heavy hydrocarbons that avoids or minimises the problem of fouling of steam generators. In particular methods are needed that do not require the use of extensive water purification treatments to enable the recycling of water recovered from hydrocarbon production for use in steam generation or significant amounts of fresh water to be regularly introduced.
It has now been discovered that these problems may be overcome by generating supercritical steam in a steam generator. The use of supercritical steam means that the heat transfer surfaces are mainly exposed to a flowing supercritical phase therefore the risk of drying out at the pipe surface with precipitate forming and sticking thereto (i.e. fouling occurring) is significantly reduced. Moreover because the steam is in supercritical form the pipes in the boiler may have a small diameter and therefore high flow rate. This additionally helps to prevent fouling and advantageously provides a large heat transfer surface for efficient heat transfer.